| General > General Technical Chat |
| LED Lighting (350 mA). Is there any standard for # of LEDs is a string? |
| << < (3/12) > >> |
| Benta:
OK. no standard for power LED strings. I can live with that, but it's a bit of a nuisance. I'm going completely KISS here, so no step-up converters or other things. This has to be as simple and cheap as possible. Also, when I say "LED string", I don't mean metre-long things with 100s of LEDs. I'm talking about white power LEDs for lighting. At this time, it seems the electrical optimum is 4 LEDs per string at 19 V supply. The laptop supply could then drive up to 13 strings (12 is probably a better number) that can be mounted optically separately or in groups. Each string will have its own simple controller. The simplest I've come up with is this using Osram OSLON SSL 150 whites, and it works like a dream, spot-on at 350 mA: |
| Zero999:
How many strings are you planning to have per luminaire? Consider having a lower reference voltage, then you can have an extra LED or two and cut down power dissipation in the transistor. |
| mariush:
You could use a basic linear driver like BCR421 which can do up to 350mA (absolute max rating somewhere around 500mA) using an external resistor (defaults to 10mA without one) : https://www.digikey.com/en/products/detail/diodes-incorporated/BCR421UFDQ-7/7930771 32 cents if you get 100, 20 cents if you buy 1000 ... datasheet here : https://www.diodes.com/assets/Datasheets/BCR420UFDQ-BCR421UFDQ.pdf Note that the enable pin can only handle up to 18v but it can also be used as pwm. The IC has a voltage drop of around 1v ... if you allow for 1.5v drop you're looking at 18v - 1.5v = 16.5v / 3.2v = 5 leds in series. If you go with a step-down led driver, you could have groups of 3 leds in parallel, so your step-down led driver would support a wider range of laptop adapters ex 12v ...18.5v or even only 16.5v .. 18.5v and output 8.5...10v (3 leds x 2.8v...3.3v in series) for example AL8843 : https://www.digikey.com/en/products/detail/diodes-incorporated/AL8843QSP-13/10668306 it can do up to 3A , so you could safely do 8 groups of leds in parallel as 8x 0.35 = 2.8A ... so then have 3-5 leds in series, then 8 such series in parallel, for a total of 24-40 leds controlled by one driver. |
| nctnico:
LEDs strings in parallel don't work well. You'll see differences in brightness. I've seen current mismatches up to 25%. For the best results, you need a current source per string (even if it is a simple dropping resistor). |
| mariush:
You can bin the leds so that the forward voltage of every series ends up very close to the forward voltage of other series that are in parallel. Super easy to make a test fixture of some sort to measure the forward voltage and brightness at various current levels. They do this parallel thing with led strips for monitor backlights and in other places and nobody complains about differences in brightness. |
| Navigation |
| Message Index |
| Next page |
| Previous page |